First you need to sign out of your account. You can do this by either clicking on the info/settings bar on the bottom right of your screen then clicking “sign out” at the top of the resulting pop-up menu, or hitting CTRL+SHIFT+Q (you have to do it twice) to sign out. Once you are signed out, on the start screen you should see an option to browse as guest. I’ve seen it both at the bottom of the start screen on the “shelf” aka taskbar and just under the login for the main user. It should be pretty obvious once you have logged out of your main account and are at the start screen. Also, make sure that your guest logs out the same way as you did to delete their browsing history (unless you want to go all stalker on them).
Here is a link to the Google video that shows the process. It is not quite the same as on my HP Chromebook 14, although I’m not sure if that’s just because Chrome has been updated since then or if it varies slightly from manufacturer to manufacturer. The basic process is still the same though. https://www.youtube.com/watch?v=HCw3SLxRNXM